In telephone, call forwarding (also referred to as call diversion) is a feature on some telephone networks which lets an incoming call to a called party (having a telephone number corresponding to the number dialed by a caller) be redirected to a third party that has a telephone number different from the dialed number (referred to herein as a destination number).
Call forwarding lets a user set up automatic forwarding of incoming calls to a particular telephony device (occasionally described herein as the target of the call) to another telephony device (the “destination device”). Such call forwarding arrangements can be across national borders, so that an international traveler can, for example, arrange for automatic redirection of calls to, e.g., her US-based mobile phone to a telephone in another country, for example to a landline telephone in her Shanghai hotel room.
Setting up such call forwarding arrangements can sometimes be hampered by a user's unfamiliarity with particular codes or digits that need to be dialed together with the destination device's local number or subscriber number (such as exit codes, area codes, country codes, and/or private branch exchange (PBX) codes, or the like) in order for successful telephony connections to be established with the destination devices.
Consider that some systems allow calls within a local area to be made without dialing the local area code. For example, a phone number in North America will start with three numbers (such as 661), which is the area code, followed by seven digits split into sections of three and four (such as 550-1212), which may be one example of a local number consistent with this description. Most telephone networks are interconnected in the international telephone network, where the format of telephone numbers is standardized by the International Telecommunication Union Telecommunications Standardization Sector (ITU-T) in recommendation E.164. This specifies that the entire number should be 15 digits or shorter, and begin with a country prefix. For most countries, this is followed by an area code or city code and the subscriber number, which might consist of the code for a particular telephone exchange. ITU-T recommendation E.123 describes how to represent an international telephone number in writing or print, starting with a plus sign (“+”) and the country code, but advertised telephone numbers do not always comply with this standard.
For example, international telephone numbers are often prefixed with the country code preceded by a “+”, and with spaces in place of hyphens (e.g., “+XX YYY ZZZ ZZZZ”). This allows the reader to choose which access code (also known as International Dialing Digit) they need to dial from their location. However, it is often quoted together with the international access code which must precede it in the dial string, for example “011” in some countries (including Canada, Bermuda, and the United States): “011-XX-YYY-ZZZ-ZZZZ”, or “00” in most European countries: “00-XX-YYY-ZZZ-ZZZZZZ”. To complicate matters further, consider that in the Global System for Mobile Communications (GSM) networks, “+” is an actual character that may be used internally as the international access code, rather than simply being a convention.
These differing conventions and codes can cause user confusion, particularly for international travelers.